The present invention relates generally to wireless communications, and more particularly, to a receiver for DFT-Spread MIMO-OFDM systems.
Referring to the diagram in FIG. 1, each mobile (or source) transmits its signal using the DFT-S-OFDM technique. The destination or base station receives signals from several mobiles possibly overlapping in time and frequency and has to decode the signal of each mobile.
Discrete Fourier Transform-spread-Orthogonal Frequency Division Multiple Access (DFT-spread-OFDMA or DFT-S-OFDMA) has emerged as the preferred uplink air interface for the next generation cellular systems such as the 3GPP LTE. The main advantage of this multiple access technique is that it results in considerably lower envelope fluctuations in the signal waveform transmitted by each user and consequently lower peak-to-average-power ratio (PAPR) compared to the classical OFDMA technique. A lower PAPR in turn implies a smaller power back off at the user terminal and hence an improved coverage for the cellular system. Another key technology that will be employed in the upcoming cellular systems is the utilization of antenna arrays at the base station (a.k.a Node-B) and possibly at the user equipment (UE). Multiple antennas when used in point-to-point or multipoint-to-point systems have been shown in theory to result insubstantial capacity improvements, provided that the environment is sufficiently rich in multipath components. However, in practice the capacity improvement obtained by using multiple antennas at the UEs in the uplink can be much smaller due to the fact that multiple antennas will have to be accommodated in the limited space available at the UE, which will result in correlated channel responses that are not conducive to high rate communications. Moreover, installing multiple power amplifiers in each UE is currently deemed impractical based on cost considerations by many vendors.
A promising scheme, also adopted in 3GPP LTE, which circumvents these two issues is the space-division multiple-access (SDMA) scheme which is sometimes referred to as the virtual multiple-input-multiple-output (MIMO) scheme. In SDMA multiple single-antenna users are scheduled over the same frequency and time resource block in order to boost the system throughput. Since different users are geographically separated, their channel responses seen at the base-station antenna array will be independent and hence capable of supporting high rate communications. Henceforth, the DFT-S-OFDM based uplink employing SDMA will be referred to as the DFT-S-OFDM-SDMA uplink.
In DFT-S-OFDM systems, which encompass both DFT-S-OFDMA and DFT-S-OFDM-SDMA, as a consequence of the DFT spreading operation at the transmitter, the signal arrives at the base-station with substantial intersymbol interference and the received sufficient statistics can be modeled as the channel output of a large MIMO system. The conventional receiver technique involves tone-by-tone single-tap equalization followed by an inverse DFT operation. While such a simple receiver suffices for the single-user case in the low-rate regime when there is enough receive diversity and where the available frequency diversity can be garnered by the underlying outer code, it results in degraded performance at higher rates as well as with SDMA.
Unfortunately, unlike classical OFDMA, the large dimension of the equivalent MIMO model in DFT-S-OFDMA does not allow us to leverage the sphere decoder which has an exponential complexity in the problem dimension. Furthermore, the stringent complexity constraints in practical systems also rule out the near-optimal MIMO receivers developed for the narrowband channels. Other promising equalizers for the DFT-S-OFDM systems are the decision feedback equalizers (DFE), in particular the hybrid DFE, where the feedforward filter is realized in the frequency domain and the feedback filter is realized in the time domain, and the iterative block DFE with soft decision feedback that has been proposed by others, where even the cancelation is performed in the frequency domain. However, even the DFE whose iterative process does not include decoding the outer code is substantially more complex and has higher latency especially in the SDMA case, than the conventional receiver.
Accordingly, there is a need for a receiver at the destination or base station that can receive and decode multiple wireless signals overlapping in time and frequency in a manner that overcomes the limitations of the conventional receiver techniques discussed above.